1. Field of the Invention
The present invention relates to a device for driving an image forming member (such as a pen or a clipping cutter) in an automatic drawing apparatus and particularly to a driving device of an automatic drawing apparatus for controlling the raising and lowering of the image forming member or an image forming pressure.
2. Description of the Background Art
A drive for an image forming member of interest to the present invention is disclosed for example in Japanese Utility Model Laying-Open No. 66534/1974 or Japanese Patent Laying-Open No. 148696/1984.
In both of the above indicated documents, the image forming member of the automatic drawing apparatus is coupled to a moving coil and it is driven vertically through control of the magnitude of current flowing in the moving coil.
According to the first document (No. 66534/1974), the magnitude of the current flowing at the time of lowering or raising the image forming member of the automatic drawing apparatus, connected to the moving coil is controlled in the following manner. When a lowering in the speed of the image forming member is detected (by detection of a quantity of light and by differentiation of a signal of the detected light quantity according to that document), the magnitude of the current that should flow in the coil is determined by subtracting the value of current which is dependent on the lowering speed from a predetermined value of current which should flow in the coil. In other words, the decrease in speed of the image forming member is detected and the value of current is controlled so that the speed is equal to a predetermined value. Thus, closed loop control is carried out solely by detecting the speed. As a result, the image forming member is allowed to soft-land on a sheet where the image is formed.
According to the second of the aforementioned document, the magnitude of the current flowing in the moving coil connected to the image forming member of the automatic drawing apparatus is controlled in an open loop by using a function generating circuit. When the image forming member is to be lowered, it is first accelerated to be lowered. The lowering speed is decreased before the image forming member reaches the image formation surface and, after that, the image forming member is stopped. Then, the image forming member is accelerated a little so that it may soft-land on the image formation sheet. After that, a predetermined pressure is applied to the image forming member to form an image through the above mentioned soft-landing, contacting of the image forming member with the image formation sheet occurs without causing any impact or damage.
More specifically, the image forming member is lowered at high speed toward the image formation sheet so as to be close thereto and is then soft-landed on the image formation sheet. As a result, any impact or damage on the image formation sheet which would be caused by the image forming member can be prevented and an image is formed with fine traces on the image formation sheet.
Generally, in an automatic drawing apparatus, the image forming member needs to be operated accurately and smoothly so as to prevent the image forming member from causing any damage and to form an image of fine lines. For those purposes, it is necessary to control the magnitude of current flowing in the moving coil connected with the image forming member accurately and smoothly. This is because the image forming member is driven in proportion to the current flowing in the moving coil based on the Flemings' left-hand rule.
The automatic drawing apparatus of the above indicated first conventional example has a speed detector for detecting a speed of the image forming member but this detector detects only the lowering speed of the image forming member. The lowering speed of the image forming member is controlled according to the detected value so that the image forming member is soft-landed on the surface of the image formation sheet. This prior art example involves disadvantages. It is possible that the lowering time or the landing point of the image forming member will differ in dependence on the distance from the image forming member to be lowered to the surface of the image formation sheet. Accordingly, the image forming member might collide with the image formation sheet or become bound thereon. Consequently, the life of the image forming member would be considerably reduced or that fine traces might not be obtained.
According to the second of the above indicated prior art example, the steps of raising and lowering the image forming member are controlled in an open loop. Accordingly, the stop position of the image forming member is not fixed stably compared with the closed loop control system. Consequently, the same disadvantages as in the case of the first example are present. In order to eliminate such disadvantages, it is necessary to adjust a drive control device for each drawing apparatus and such adjustment requires a lot of labor.
In each of the two examples, a current of a prescribed value is caused to flow in the moving coil after the image forming member has landed on the image formation sheet. As a result, the image forming member is pressed against the image formation sheet with a prescribed pressure. However, if a knife or similar cutter is used as the image formation member and a peel off film is used as the image formation sheet, the below described problems occur. If the peel off film is to be cut by using the knife, it is necessary to set the pressure on the knife to between 10 to 20 grams. When the peel off film is cut with such a very weak pressure for image formation, the image forming member is affected by irregularities on the image formation table due to errors of finishing of the table. Accordingly, in an extreme case, the knife will spring up and normal cutting will not be carried out.